Various techniques for removing ions from liquid streams, particularly water, have long been known in the art. For example, various filters, such as those using charcoal and ion exchange resins have been used to remove, and in some cases exchange, ions along with extraneous minerals.
For example, U.S. Pat. No. 3,213,016 to Gowers et al discloses an apparatus and technique for removing cations and/or anions from liquids, such as water, by passing the liquid containing the ions to be exchanged through a network of ion exchange resin fibers. Techniques also exist for replacing the ion exchange resin granules in a purification system that does not involve opening up the reactor, as in U.S. Pat. No. 3,378,339 to Yamashiki. One drawback to using ion exchange resins is that they must be rejuvenated periodically. Charcoal absorbers must generally be replaced entirely. U.S. Pat. No. 4,219,414 to Crits also presents an apparatus and technique for cleaning out portions of spent ion exchange means from a reactor in use. A multi-step process for exchanging ammonia and ammonium ions with the hydrogen ions of an ion exchange media is revealed in U.S. Pat. No. 3,475,330 to Gilles.
Extremely clean, deionized (DI) water is used by industry in many applications. For example, in the semiconductor industry, DI water is used for many cleaning functions such as in the high pressure jet scrubbing of semiconductor masks. However, it has been discovered that if completely deionized water is pressurized and pumped through a nozzle to form a scrubbing jet stream, the high velocity of the DI water moving through the air will strip electrons from the surrounding air molecules causing an arc to the nearest grounded surface. This phenomenon is known as the "Corona effect". Since the semiconductor mask being cleaned has chrome on its surface, the mask itself serves as the nearest available ground. Unfortunately, the high voltage arcing "burns" the chrome surface off the semiconductor mask at the area of contact rendering it useless.
As the above-described mask cleaning procedure is used industry-wide, this arcing problem is also well known. It is generally recognized that the solution is to reincorporate ions into the DI water to some extent. Using non-deionized water is an unacceptable answer because such water typically contains other mineral impurities which can damage the mask.
One technique for reintroducing ions into a DI water stream prior to the semiconductor mask scrubbing step was to employ an in-line magnesium ion exchanger. The exchanger was simply a section of pipe containing magnesium rods having their longitundial axes oriented parallel with the pipe. It was mounted in the DI water stream prior to the mask scrubber. In operation, the DI water would flow along the sides of the magnesium rods. The magnesium rods were found to be a major source of contamination both of particulates and residues (e.g. oily substances), however, particularly after the DI water was treated with peroxide to eliminate bacteria. Bacteria are also harmful to the semiconductor mask surfaces. The magnesium treatment has been replaced to some extent by a batch treatment reionizer. The batch reionizer is basically a container with a water-level sensor which allows water to flow into the container to a certain level before it shuts off. The container is pressurized with a gas, typically C0.sub.2, which is absorbed by the water. The treated water remains stagnant in the container until it is used by the scrubbing equipment, which is followed by another filling and treatment cycle. While satisfactory reionized water is produced by this latter method, the batch characteristic which permits the water to remain stagnant for a time is undesirable because it permits bacteria to form while the water remains stagnant. Both of these ionization techniques have been known to be sold by Ultratech Corp.
Thus, the various apparatus and methods known to be available to introduce ions into water are unsatisfactory, particularly when used to solve the arcing problem associated with semiconductor mask scrubbing.